Diaminopyridines and substituted diaminopyridines are well known for their use in the composition of hair dye and in the synthesis of substituted imidazole derivatives. In the dying of hair, a dye intermediate is used, which upon oxidation, develops to form the respective dye. Oxidation dyes have acquired substantial significance for hair coloration.
There are few methods known for producing diaminopyridines and substituted diaminopyridines and the use thereof in the preparation of hair dye and in the synthesis of substituted imidazole derivatives.
German patent DE 3233540 discloses oxidation hair dye compositions, which consist of 2,3-diamino-6-methoxy pyridine and aromatic amines or diamines as precursors.
U.S. Pat. No. 3,200,040 discloses a composition suitable for use in the oxidative dying of hair, which comprises 6-methoxy-2,3-diaminopyridine, as a dye intermediate.
Voker (Seifen-Ole-Fette-Wachse, 1991, 117(4), 133–136) has investigated about 100 pyridines for their suitability as hair dyeing material and it is reported that 2,3-diamino-6-methoxypyridine dihydrochloride is one of the most potential oxidative dye couplers.
U.S. Pat. No. 226,216 discloses dye compositions, comprising one oxidation base and at least one coupler chosen from 6-alkoxy-2,3-diaminopyridine derivatives.
JP Patent No. 9241259 discloses the process for the preparation of 2,3-diamino-6-methoxypyridine and use thereof in the synthesis of 2-mercaptoimidazole condensed ring compound. The process comprises of the preparation of 2,3-diamino-6-methoxypyridine by the catalytic reduction of 2-amino-6-methoxy-3-nitropyridine. The catalytic reduction is not found suitable for commercial production due to catalyst leaching and poisoning. In addition, the free base obtained is highly colored.
U.S. Pat. No. 226,216 discloses a process for the preparation of 6-alkoxy-2,3-diaminopyridine derivatives, where 6-alkoxy-3-nitro-2-halopyridine derivative is reacted with various amines in a polar solvent chosen from alcohols, acetic acid, formic acid, dioxane and dimethyl formamide at a temperature of 75°–140° C. to obtain the corresponding 2-amino-3-nitropyridine derivative. The reduction of 2-amino-3-nitro-6-alkoxypyridine derivative is performed by hydrogenation catalyzed with palladium on carbon. This approach is not found suitable for 2,3-diamino-6-methoxypyridine.
In Aust. J. Chem., 1982, 35, 2025–34, Deady et al. has reported the route for the preparation of 2-amino-3-nitro-6-methoxypyridine, the precursor of 2,3-diamino-6-methoxypyridine. In this method, 2-amino-6-methoxypyridine was nitrated with potassium nitrate and sulfuric acid. The use of potassium nitrate is not advisable at commercial scale due to its high cost and hazardous nature. In addition, the starting material 2-amino-6-methoxypyridine is also very expensive.
EP 735025 discloses the synthesis of 6-methoxy-2-amino-3-nitropyridine by amination of 6-methoxy-3-nitropyridine by using methoxyamine, potassium-tert-butoxide and zinc chloride. The raw materials used in this process are very costly and difficult to handle.
In view of the increasing demand for 2,3-diamino-6-methoxypyridine and the lack of a robust economically viable commercial process to make that compound, there is a need to develop a commercially and economically sound process.